Channel reciprocity, i.e. characteristics of frequency symmetry for uplink/downlink, have a bright prospect of application in TDD (Time Division Duplex) system for LTE-A (Advanced-Long Term Evolution). The hypothesis of reciprocity of uplink/downlink is widely recognized in the industry, and is utilized to conduct channel estimation effectively. However, in practical application, due to the difference of RF Circuit between receiver and transmitter, especially the difference of RF Circuit between receiver and transmitter on base station side, hence, assurance of reciprocity between uplink and downlink in TDD system is difficult to be kept. For a system with a reciprocity between uplink and downlink based on TDD, system performance is sensitive to the tolerance between uplink and downlink channel, and even a little tolerance between uplink and downlink channel might result in a most serious performance degradation. Thus, reciprocity calibration of the TDD system draws attention of the industry.
Without the requirement for extra hardware equipment, the calibration of an OTA (over-the-air) interface of the TDD system with a reciprocity between downlink and uplink turns out to be an effective approach to guarantee reciprocity of uplink and downlink channel of the TDD system.
R1-094623 proposed an OTA calibrating method for TDD (Time Division Duplexing) CoMP systems, which can eliminate the requirement of the resource-consuming reciprocity calibration for each transceiver of a CoMP eNB. But analysis in R1-101016 shows that the quality of OTA calibration is limited by the user equipment's (UE) channel quality. In CoMP, the UE needs to measure all its channels from/to all the CoMP eNBs relevant thereto. With merely a portion of the above channels, if not all, calibration effect would be poor, thus directly resulting in degradation of CoMP performance.
Additionally, for current CoMP, the conventional over-the-air calibration requires the participating UE(s) to be able to accurately measure the downlink and uplink channels to all eNBs in the CoMP cluster to which the UE belongs. This requirement proves to be difficult in CoMP practice because the UE could be at the edge of at least one CoMP cell if not at the edge of all the CoMP cells. That may mean poor calibration performance of the air interface channels due to the inaccurate measurement.
Therefore, there still exist some issues in the prior art, such as how to preferably select UE(s), how to configure the selected UE(s) to calibrate the antenna(s) configured on the CoMP base station. Especially for a CoMP TDD system, there is still needed a solution about UE-selecting and calibration for reciprocity of the matrix channel between the selected UE(s) and the CoMP base station related thereto.